Imu has nothing to do with Harald’s reasoning in that moment. Harald’s rationale for forcing Loki to eat the fruit was already made explicit. First, Harald felt a massive surge in his own power. Second, the fruit’s reputation is tied to world destruction. From Harald’s point of view, no one in Elbaf could ever stop him anymore unless it was Loki armed with the fruit’s power. Immortality was not the sole issue here, Harald’s raw strength was.
The story then resolves this in a very straightforward way. Loki dominates Harald in a clean 1v1, which directly validates Harald’s rationale. Loki possessing the aCoC potency required to actually kill Harald ends up being coincidental.
Only afterward does the narrative clarify the mechanics. Both Gaban and Loki later (present day) confirm that aCoC is the actual answer for the Holy Knights. That understanding came specifically from confronting Harald. Gaban even explicitly states that the Holy Knights are not truly immortal in this context. Loki himself only comes to realize that his aCoC was the decisive factor, and Harald was the only immortal opponent Loki had ever fought.
Tl:dr
Solution to standing up to an AMPED HARALD, one of the strongest fighters in existence = Legendary devil fruit with a world-ending reputation
Solution to immortality = potent enough aCoC
The story then resolves this in a very straightforward way. Loki dominates Harald in a clean 1v1, which directly validates Harald’s rationale. Loki possessing the aCoC potency required to actually kill Harald ends up being coincidental.
Only afterward does the narrative clarify the mechanics. Both Gaban and Loki later (present day) confirm that aCoC is the actual answer for the Holy Knights. That understanding came specifically from confronting Harald. Gaban even explicitly states that the Holy Knights are not truly immortal in this context. Loki himself only comes to realize that his aCoC was the decisive factor, and Harald was the only immortal opponent Loki had ever fought.
Tl:dr
Solution to standing up to an AMPED HARALD, one of the strongest fighters in existence = Legendary devil fruit with a world-ending reputation
Solution to immortality = potent enough aCoC
